1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to a novel interference microscope objective with a folded optical path to reduce the physical length of the objective.
2. Description of the Related Art
It is desirable to provide interferometric instruments with the flexibility of operating at different magnifications. Accordingly, interferometers used to measure surface roughness are often equipped with a rotatable turret capable of accommodating multiple interference microscope objectives with different optical power. In such a configuration, it is very important that the objectives be substantially parfocal, as defined below, to minimize the need for focussing after switching from one objective to another.
In surface profilometry applications, interference microscope objectives are typically used with magnifications in the 1.5.times. to 10.times. range. Based on current commercial availability, microscope objectives commonly used for these applications are used with a tube lens with a specific reference focal length (for example, typically 200 mm for Nikon, 180 mm for Olympus, and 160 mm for Zeiss objectives). Therefore, the focal length of these objectives varies inversely with their power according to the relation f.sub.objective =f.sub.Reference /power. For example, a 50.times. Nikon objective has a focal length of 4 mm; a 20.times. Nikon objective of 10 mm; and so on. Obviously, low power objectives have a relatively long focal length, which makes it difficult to design them with a physical length sufficiently short to fit alongside high power objectives that may be less than half their length. The problem is further exacerbated by the necessity of introducing a beamsplitter between the objective's lens and the test sample for interference purposes.
In practice, objectives in the 2.5.times. to 50.times. range are substantially similar in size and can be mounted coplanarly on a turret such that the distance between their mounting shoulder and their focal point is the same. For the purposes of this disclosure, multiple objectives characterized by this condition are defined as "parfocal" objectives. Referring to the Wyko 2.5.times. microscope objective 10 shown in FIG. 1, for example, which includes a beamsplitter for interference purposes, the distance D between its mounting shoulder 12, which in operation abuts the mounting plane 14 of the objective turret 16, and its focal point F (shown coincident with a sample surface S) is approximately 48 mm. The corresponding distances for comparable Wyko 5.times., 10.times., 20.times. and 50.times. objectives are about 49, 45, 45 and 45 mm, respectively.
Therefore, these reference objectives are readily rendered parfocal by adding collars of different lengths to each one to provide a common nominal parfocal distance D' to the focal point F. This modification is illustrated in the Wyko 10.times. objective 20 shown in FIG. 2, also including a beamsplitter for interference purposes, where an extension collar 22 is inserted between the shoulder 12 of the objective and the plane 14 of the turret 16 so as to increase the distance D by about 6.35 mm to a nominal parfocal distance D' of approximately 51.35 mm. Using this approach, all objectives with powers in the range between 2.5.times. and 50.times. are modified in a relatively simple manner for parfocal use in a single turret having a mounting plane 14 that produces a substantially focussed condition when the object under test is placed at about the distance D' from the plane 14. Thus, once an object is in focus for one objective, it remains substantially so when the turret 16 is rotated and other objectives are placed in operation.
For the purpose of this disclosure, the focal point F is assumed to be coincident with the location of the sample surface S. This describes the "infinite conjugates" imaging condition, in which the objective forms an image of the sample an infinite distance above the objective. This image then acts as an object for the tube lens, which forms a (typically) magnified image of the sample at a fixed location in space. As one skilled in the art would readily understand, though, the disclosure applies equally to the case where the focal point is above the sample surface, and an image of the surface is formed a finite distance above the objective without the use of a tube lens. Thus, while the "infinite conjugates" imaging condition is used here to simplify the description of the invention, the description can be generalized with no loss of accuracy to the "finite conjugates" imaging condition. It can also be applied to the case where the focal point is below the sample surface S.
Unfortunately, 1.5.times. and 2.times. objectives have a relatively long focal length that in practice is not suitable for the same solution. For example, the distance D for Wyko 1.5.times. and 2.0.times. objectives is about 115 mm, about 64 mm too long for fitting within the nominal parfocal distance D' of 51.35 mm used for the more powerful, and shorter, objectives. Thus, it is clear that low-power objectives could not be mounted parfocally on a turret alongside more powerful objectives. If sufficient space were provided to mount both, focussing a "short" objective would cause an adjacent "long" objective on the turret to physically contact the test sample or the microscope stage. The same could happen while rotating the turret to place the long objective in operation. It is noted that for the purpose of this disclosure "low-power" objectives are considered to be those with magnifications of 1.5.times. and 2.times., while "high-power" objectives are those with magnifications of 2.5.times. to 50.times.. Although these definitions do not correspond to normal microscopy usage, where high power typically would refer to magnifications somewhat greater than 2.5.times., they are appropriate for the description of the invention, which regards the physical length of the objectives in relation to their power of magnification.
Several practical difficulties prevent the design of a turret/microscope assembly with sufficient height D' to accommodate all objectives of interest in a substantially parfocal condition (that is, theoretically, very tall collars 22 could be used with high-power objectives to obtain the same nominal parfocal distance D' for all objectives). One problem with this approach is that degraded illumination would result for the more powerful objectives due to a substantial misalignment along the optical axis of their entrance pupils with respect to the less powerful objectives. This misalignment of the entrance pupils would also cause the light beam to pass through different portions of the collection optics for the high-power versus the low-power objectives. In addition, the telecentricity of microscope objectives, which prevents changes in magnification during focussing, makes it difficult to substantially shorten the length of the low-power objective in a practical and economically feasible manner. Finally, the problem is further complicated by the fact that interference objectives require a beamsplitter in the optical path. Since Michelson interferometers, which employ a cube beamsplitter placed between the last lens and the test object, are most suitable for low-magnification objectives, the additional space occupied by the beamsplitter further decreases the length available for placement of the other optical components of the objectives while maintaining a given parfocal length.
Accordingly, there is still a need for a low-power interference objective that is substantially commensurate in length with higher-power objectives for concurrent use mounted in parallel on a rotatable turret. This invention is directed at providing a novel approach to that end.